© 2014 Pearson Education, Inc. Thermochemistry Klein Forest High School-Chemistry Jarrod G Collins, M.Ed.

© 2014 Pearson Education, Inc. What are the ways in which energy changes can occur? Thermochemistry is the study of energy changes that occur during chemical reactions. –During a chemical reaction, a substance is transformed into another substance with a different amount of chemical potential energy. –Energy changes occur as either heat transfer or work, or a combination of both.

© 2014 Pearson Education, Inc. Heat Exchange Heat is the exchange of thermal energy between a system and surroundings. Heat exchange occurs when system and surroundings have a difference in temperature. Temperature is the measure of the average kinetic energy of the particles (thermal energy) Heat flows from matter with high temperature to matter with low temperature until both objects reach the same temperature. –Thermal equilibrium

© 2014 Pearson Education, Inc. Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between the objects. –One of the effects of adding heat to an object is an increase in its temperature. –Heat flows spontaneously from a warmer object to a cooler object. –If two objects remain in contact, heat will flow from the warmer object to the cooler object until the temperature of both objects is the same. Heat 1. Describe An ice cube at 0ºC is dropped into a cup of water at 45ºC. Describe what happens to the ice and water in terms of heat flow.

© 2014 Pearson Education, Inc. Energy in Chemical Reactions

© 2014 Pearson Education, Inc. Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances Nuclear energy is the energy stored within the collection of neutrons and protons in the atom Electrical energy is the energy associated with the flow of electrons Potential energy is the energy available by virtue of an object’s position

© 2014 Pearson Education, Inc. Conservation of energy The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. –During any chemical or physical process, the energy of the universe remains unchanged. –If the energy of the system increases during a process, the energy of the surroundings must decrease by the same amount. –If the energy of the system decreases during a process, the energy of the surroundings must increase by the same amount.

© 2014 Pearson Education, Inc. Transfer of Energy Two Ways to Transfer Energy: –Heat- (q) transfer of energy between two objects because of a temperature difference –Work- (w) force acting over a distance

© 2014 Pearson Education, Inc. Potential Energy: Potential Energy is stored energy. It is energy something has because of its position or composition. What has more potential energy – a bowling ball on the ground, or a bowling ball three feet directly above your head? What has more potential energy – a barrel of water or a barrel of gasoline?

© 2014 Pearson Education, Inc. Chemical potential energy That’s the type of energy that gasoline has. Defined as the potential energy stored in the bonds between atoms of a substance.

© 2014 Pearson Education, Inc. Chemical potential energy When you burn gasoline, or wood, or anything flammable, much of the chemical potential energy is released as heat.

© 2014 Pearson Education, Inc. Chemical potential energy Burning isn’t the only chemical reaction that releases heat. Virtually every chemical reaction either releases heat (exothermic) or absorbs heat (endothermic). Exothermic: heat is exiting.

© 2014 Pearson Education, Inc. Endothermic process Heat is absorbed from the surroundings in an endothermic process. –In an endothermic process, the system gains heat as the surroundings lose heat. –Heat flowing into a system from its surroundings is defined as positive; q has a positive value here.

© 2014 Pearson Education, Inc. Exothermic process An exothermic process is one that releases heat to its surroundings. –In an exothermic process, the system loses heat as the surroundings gain heat. –Heat flowing out of a system into its surroundings is defined as negative; here, q has a negative value.

© 2014 Pearson Education, Inc. Transfer of Energy system- part of the universe you are focused on surroundings- everything else in the universe usually – system: what is inside the container –surroundings: room, container, etc.

© 2014 Pearson Education, Inc. System? Surroundings? The “system” is generally the container where the reaction is taking place.

© 2014 Pearson Education, Inc. Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. If energy is listed as a product, the reaction is exothermic: Endothermic process is any process in which heat has to be supplied to the system from the surroundings. If energy is listed as a reactant, the process is endothermic: 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy energy + 2HgO (s) 2Hg (l) + O 2 (g) Reactants are A+B, Products are C Reactants are A+B, Products are C+D

© 2014 Pearson Education, Inc. Look at the (PE) potential energy, where it is determines whether it is an endo or exothermic reaction. If the (PE) lower, its exothermic and negative If the (PE) higher, then its endothermic and positive To find values, subtract products minus reactants on graph

© 2014 Pearson Education, Inc. Endothermic or Exothermic? (how to decide) Fires give off heat, and feel hot to your hands. They are exothermic. Cold packs feel cold to your hands, so they must be the opposite: endothermic. They feel cold because they’re absorbing heat from your hands.

© 2014 Pearson Education, Inc. Chemical energy Is this reaction exothermic or endothermic?

© 2014 Pearson Education, Inc. Potential energy Heat Is this reaction exothermic or endothermic?

© 2014 Pearson Education, Inc. Chemical energy What about this one?

© 2014 Pearson Education, Inc. Potential energy Heat What about this one?

© 2014 Pearson Education, Inc. Practice Questions Is this reaction endothermic or exothermic? What is the change in energy for the reaction?

© 2014 Pearson Education, Inc. Practice Questions Is this reaction endothermic or exothermic? What is the change in energy for the reaction? Subtract products minus reactants Products = 100 Reactants = = 50 Value is positive, so Endothermic

© 2014 Pearson Education, Inc. Practice Questions Does the graph represent an endothermic or exothermic reaction? What is the change in energy for the reaction?

© 2014 Pearson Education, Inc. Calorimetry We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. In calorimetry... –the heat released by the system is equal to the heat absorbed by its surroundings. –the heat absorbed by the system is equal to the heat released by its surroundings. The total heat of the system and the surroundings remains constant.

© 2014 Pearson Education, Inc. Calorimetry Constant-Pressure We use an insulated device called a calorimeter to measure this heat transfer. calorimeter- device used to experimentally find the heat associated with a chemical reaction substances respond differently when heated

© 2014 Pearson Education, Inc. Quantity of Heat Energy Absorbed: Heat Capacity When a system absorbs heat, its temperature increases. The increase in temperature is directly proportional to the amount of heat absorbed. The proportionality constant is called the heat capacity, C. –Units for heat capacity, C are J/°C or J/K. The larger the heat capacity of the object being studied, the smaller the temperature rise will be for a given amount of heat.

© 2014 Pearson Education, Inc. Factors Affecting Heat Capacity The heat capacity of an object depends on its amount of matter. –It is usually measured by its mass. –200 g of water requires twice as much heat to raise its temperature by 1 °C as does 100 g of water. The heat capacity of an object depends on the type of material. –1000 J of heat energy will raise the temperature of 100 g of sand 12 °C, but only raise the temperature of 100 g of water by 2.4 °C.

© 2014 Pearson Education, Inc. Specific Heat Capacity Measure of a substance’s intrinsic ability to absorb heat. The specific heat capacity is the amount of heat energy required to raise the temp of one gram of a substance 1 °C. - C s -Units J/(g ∙ °C)

© 2014 Pearson Education, Inc. Quantifying Heat Energy The heat capacity of an object is proportional to the following: –Its mass –The specific heat of the material So we can calculate the quantity of heat absorbed by an object if we know the mass, the specific heat, and the temperature change of the object.

© 2014 Pearson Education, Inc. Example How much energy is needed to heat 80 grams of water from 10 o C to 55 o C at constant pressure? Specific heat of water (C H 2 0 ) = J/g o C q = m C T = m C (T final – T initial ) q = (80grams) ( J/g o C) (55 o C – 10 o C) q = joules divide by 1000 to get kilojoules J 1 kJ = q = 1000J kJ absorbed

© 2014 Pearson Education, Inc. Calculating Heat and Heat Capacity Practice problem 1.5 g of copper was heated from 20  C to 80  C. How much energy was used to heat the Cu? Specific heat of Cu (C Cu ) = 0.38 J/(g  C)

© 2014 Pearson Education, Inc. Enthalpy The heat lost or gained by a system during a physical or chemical change is called the enthalpy change (ΔH) or the heat of reaction. Remember that a negative enthalpy change means that heat is lost from the system to the surroundings, making the process exothermic. A positive enthalpy change tells us that heat is gained by the system and the process is endothermic.

© 2014 Pearson Education, Inc. Enthalpy The enthalpy, H, of a system is the sum of the internal energy of the system. The enthalpy change,  H, of a reaction is the heat evolved in a reaction at constant pressure.  H reaction = q reaction at constant pressure When  H is negative, heat is being released by the system. –This is called an exothermic reaction. (-q) When  H is positive, heat is being absorbed by the system. –This is called an endothermic reaction. (+q)

© 2014 Pearson Education, Inc. Practice Question Which statement correctly describes an endothermic chemical reaction? (1) The products have higher potential energy than the reactants, and the ΔH is negative. (2) The products have higher potential energy than the reactants, and the ΔH is positive. (3) The products have lower potential energy than the reactants, and the ΔH is negative. (4) The products have lower potential energy than the reactants, and the ΔH is positive.

© 2014 Pearson Education, Inc. Practice Question What is  for this reaction? Number and sign)

© 2014 Pearson Education, Inc. Thermochemical Equations H 2 O (s) H 2 O (l)  H = 6.01 kJ Is  H negative or positive? System absorbs heat; the system will feel cold. Endothermic  H > kJ are absorbed for every 1 mole of ice that melts at 0 0 C and 1 atm. 6.4

© 2014 Pearson Education, Inc. Thermochemical Equations CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (l)  H = kJ Is  H negative or positive? System gives off heat Exothermic  H < kJ are released for every 1 mole of methane that is combusted at 25 0 C and 1 atm. 6.4

© 2014 Pearson Education, Inc. H 2 O (s) H 2 O (l)  H = 6.01 kJ The coefficients always refer to the number of moles of a substance Thermochemical Equations If you reverse a reaction, the sign of  H changes H 2 O (l) H 2 O (s)  H = kJ If you multiply both sides of the equation by a factor n, then  H must change by the same factor n. 2H 2 O (s) 2H 2 O (l)  H = 2 x 6.01 = 12.0 kJ 6.3